Predicted Information Content of Gaia-Hipparcos

TL;DR

This paper analytically predicts Gaia-Hipparcos parallax errors, highlighting improved accuracy for certain stars, especially those in the Hipparcos subset, which is crucial for specific stellar classes like RR Lyrae.

Contribution

The paper provides an analytical model predicting Gaia's parallax error performance based on Hipparcos data, emphasizing the improved accuracy for a subset of stars.

Findings

Gaia parallax errors are predicted to be around 20 microarcseconds for certain stars.

Hipparcos subset stars have significantly lower positional errors, enhancing measurement precision.

The predictions will serve as an early test for Gaia's data processing algorithms.

Abstract

The Gaia-Tycho release, scheduled for 14 September, is forecast to yield parallax errors of $\sigma(\pi)\sim 300\,\mu$as for about 2 million Tycho stars. We show analytically that the actual performance should be $$ \sigma(\pi) = {\rm max}(\sigma_{1991}/96,20\,\mu{\rm as}) $$ where $\sigma_{1991}$ is the positional error from the Hipparcos mission. For typical Tycho stars, $\sigma_{1991}\sim 30\,$mas, so this reproduces the usual claims. However, for the 100,000 star Hipparcos subset of this sample, $\sigma_{1991}$ is a factor 15 or more smaller. These much lower Hipparcos positional errors apply even to stars at the Hipparcos-Tycho limit, $V\sim 12$. This is especially important for RR Lyrae stars, as well as other special classes, that were systematically included in the Hipparcos catalog down to this limit because of their exceptional scientific importance. This predicted performance will provide an early test of the Gaia algorithms.